Mouse genome contains at least 50 copies of murine leukemia viral (MuLV)-related DNAs, the majority of which are defective. Recombination between different infectious and defective MuLV sequences results in the generation of novel MuLVs including lymphomagenic mink cell focus-forming (MCF) viruses. The main goals of this project are: 1) to study the structure and regulation of expression of endogenous defective MuLV proviral DNAs; 2) to study generation of recombinant MCF viruses; and 3) to identify sequences contributing to leukemogenicity of MCF MuLVs. Full-length (8.4 kb) transcripts of endogenous MCF env-related proviruses were highly-expressed in thymus tissues of autoimmune mouse strains but were not detectable in nonautoimmune strains. The 8.4 kb RNA species was detected from day 1 of life in NZB mice, prior to clinical manifestations of autoimmune disease. The data indicates that the expression of endogenous MCF-related DNAs is regulated differently in autoimmune versus non- autoimmune mouse strains. An MCF LTR-specific DNA probe was constructed to identify MuLV proviruses which could donate LTR sequences to generate recombinant MCF MuLVs. Two potential LTR-progenitors were detected; one contained xenotropic env-related sequences and was chromosomally mapped as the inducible, endogenous xenotropic MuLV locus, Bxv-1; the second was characterized as a novel endogenous MuLV provirus which was related in env to MCF MuLVs. The latter MuLV DNA was conserved in the genomes all inbred and several wild mice. To study the contribution of the integrase gene in MCF-induced thymomas, a 2.7 kb DNA segment containing the 3' pol region of leukemogenic MCF13 was substituted by an analogous segment from non-leukemogenic MCF111A MuLV DNA. The recombinant virus obtained from transfection studies was injected into newborn AKR mice to test whether the MCF viral genome retained its leukemogenic potential with the substituted integrase gene. Tumor development was delayed and the incidence of thymomas was reduced by 50%. These results suggest that the integrase gene contributes to leukemogenicity of MCF viruses.